EXPLORATION FOR GOLD IN A
RESISTIVE ENVIRONMENT - A CASE STUDY FROM DONA (EAST), JONNAGIRI SCHIST BELT,
K. P. R. Vittal Rao, K. V. Satyanarayana, S. Srinivas and R. S. Acharya
WHERE AND HOW OF PROSPECTING
FOR DIAMONDS IN ANDHRA PRADESH, SOUTH INDIA.
V. Babu Rao and T. Srinivasulu
SEISMIC REFRACTIONS SURVEY OF
THE YESHWANTSAGAR RESERVOIR EMBANKMENTS FOR AUGMENTATION OF WATER SUPPLY TO
R. S. Ramteke, N. Ghosh, C. Subba Rao and M. S. Satpute
Induced polarization methods are generally successful in identifying sulphide associated auriferous zones when the host rock and the surrounding rock units have a good contrast in conductivity. Since I P phenomenon is basically related to electrochemical activity. I P effects are dependent on the porosity of the rock units besides the two types of conduction, electrolytic and electronic, normally present in any mineralized rock. In Dona (east) block gold mineralization is associated with highly sheared, altered granodiorite traversed by quartz veins with sparse sulphide disseminations. As the country rock granodiorite and the host rock are 'resistive' reducing the porosity considerably, the I P signals are poor. However, the alterations associated with the sheared rock enabled its delineation recording high resistivities. The sulphide rich zones are reflected as chargeability 'highs'. The S P response, though feeble, probably relates to fractured zones. A few borehole sections alongwith geophysical profiles are discussed to exemplify the above geological situation.
Exploration for Diamonds is synonymous with the exploration for Kimberlite and Lamproites which were recognized as the primary source rocks for the Diamonds. Andhra Pradesh is known as "Home of Diamonds" since historic times as world famous Diamonds like Kohinoor, Orloff and Pitt were won from the gravels of Krishna valley. The economic value of the Diamonds, needs of the domestic industry and the spate of recent discoveries of primary sources in Andhra Pradesh call for a relook into the probable areas in Andhra Pradesh.
The exploration for kimberlites and lamproites is dependent on the geological environment and requires multi-disciplinary and integrated approach. The rarity and the smallness of the kimberlite diatremes and dykes and the wide range of variation of their physical properties make the exploration for kimberlites a challenging task. The role of geophysics in diamond exploration is discussed in the light of the past experience.
Based on the aeromagnetic survey over parts of Cuddapah basin and the adjoining crystallines, a couple of dipole anomalies located within the Cuddapah basin which could be due to suspected kimberlites along with a few other prospective areas both within and outside the Cuddapah basin are recommended for ground checks.
Seismic refraction survey was carried on the downstream side of the embankment of Yeshwantsagar reservoir near Indore, M. P., to delineate the bedrock profile and to detect weak zones. If any, for founding the structure of new spillway. Four continuous seismic profiles of length varying from 105 m to 405 m were taken. The results of the seismic survey revealed a three layer earth section. Seismic wave velocity through the overburden ranged from 500 m/sec to 800 m/sec and it comprises mainly soft clay. The second layer, comprising stiff clay/coarse sand with gravel has a compressional wave velocity ranging from 1400 m/sec to 2500 m/sec. The third layer, being compact basalt, has a compressional wave velocity ranging from 4000 m/sec to 5200 m/sec indicating good quality of rock. Rock having compressional wave velocity less than 4000 m/sec was considered to be weak, weathered or fractured. The depth to the bedrock ranged between 6.7 m to 17.3 m and was corroborative with borehole data.
The purpose of the 2.5D acoustic wave simulation is to find out the 3D wave propagation effects with the help of two dimensional parameters. Linear (1991) developed a 2.5D acoustic wave equation using 3D Green's function for a constant density medium. The 2.5D simulation is of great importance because of the prevalence of the 2D survey in the seismic exploration and the cost of the 3D seismic modelling. This paper presents the development of 2.5D finite difference algorithm based on Liner's equation as well as the simulated results and study of various numerical artifacts. Comparisons of second as well as fourth order accurate 2.5D with 2D acoustic wave simulation results have been given. The snapshot at different times and zero off-set response of Sponge transmissive as well as Clayton and Engquist (1980) absorbing boundary conditions have been studied. After the extensive analysis it has been found that the stability condition and the requirement of number of grid points per - wavelength to avoid grid dispersion is the same as for the 2D acoustic cases.
The north India is one of the seismically active regions of the world. Two very high magnitude earthquakes occurred in this region in 1897 (M=8.7) and 1950 (M=8.7) during last one hundred years. History of the region indicates occurrence of destructive earthquakes at the interval of about 50 years. Applications of b-value and Gumbel's statistical methods to the earthquakes of magnitude 8 or greater. The last big earthquake of magnitude 8.7 occurred in 1950. So, statistically the region is ready for an earthquake of similar magnitude. Diminution of the number of high magnitude earthquakes since 1960, diminution of ts/tp, ratio with time and accumulation of strain in the region tendto indicate the probability of occurrence of a high magnitude earthquake in the region in the near future.